This blog post has been written by Prof. Ian Lyon
Tiny crystals of zircon (ZrSiO4) are found in many rocks and have become the mineral for measuring the ages of rocks. This is because zircons contain relatively high amounts of uranium and thorium which decay to lead (Pb) and zircon remains stable even under conditions of high temperature and pressure as occurs during rock metamorphism. Age dating of zircon has thus become the predominant method for determining the age of events which occurred throughout the 4.5 billion years of Earth history. To understand why analysing lead isotope ratios can give age dates, we must look at the radioactive decay of uranium.
Uranium is composed of 2 radioactive isotopes, 235U with a half life of 710 million years and 238U with a half life of 4500 million years. Both have complex decay chains but 235U decays to 207Pb and 238U decays to 206Pb, both of which are stable. In addition 232Th decays to produce 208Pb. Uranium that occurs naturally in the Earth today contains 99.3% 238U and 0.7% 235U and since 235U is the important ‘fissile’ isotope of uranium that powers nuclear reactors and bombs, the efforts of actual and aspiring nuclear-capable states are devoted to enriching the 235U content of uranium to much higher levels than the present day 0.7%. Highly enriched uranium might contain up to 30% 235U and is technically very difficult to achieve today. In the past however, because 235U has a half life of ‘only’ 710 million years which is only a fraction of the age of the Earth, there was much more 235U in the Earth than there is today. Indeed, if we go back more than 2 billion years in Earth history, naturally occurring uranium in the Earth was what we would today call highly-enriched uranium and decaying to produce far more 207Pb than uranium will today. This leads to a very elegant result that the 207Pb/206Pb ratio measured in the zircon directly gives the age at which it formed.
So far so good and this is where zircon shows its strength as a great mineral for age dating because even if it is buried and heated to >1000°C it can retain its Pb, and even if Pb is partially lost from the zircon, in theory the 207Pb/206Pb ratio remains the same, retaining the information on its formation age.
Very recently however, it was discovered that when the zircon was heated, the Pb can move around and doesn’t remain dispersed throughout the zircon, but can coagulate into tiny nanometer sized Pb spheres. Provided all of the Pb in the zircon is measured together, the formation of Pb nanospheres doesn’t make any difference for determining an age but since more spatially resolved techniques have been developed, people have been finding that they can get different results at different points on the zircon and the ages determined are less than reliable.
The nanospheres truly are only a few nanometers in diameter and composed of Pb metal. The challenge was to measure the 207Pb/206Pb ratio of the Pb atoms in such tiny nanospheres which might have less than 100,000 atoms of Pb. This was done by using a mass spectrometer called a NanoSIMS which fires a focused beam of oxygen ions at the zircon and nanospheres and separates the Pb ions produced into 207Pb and 206Pb.
The figure below shows isotope maps produced by the NanoSIMS of the zircon showing the Pb nanospheres and from which 207Pb/206Pb ratios could be determined.
Because the nanospheres were formed billions of years ago and isolated from inputs of further Pb since then, the nanospheres retain the 207Pb/206Pb ratio frozen into them at the time at which they formed and so represent an isotopic ‘fossil’ of that distant time. Uranium decay in the zircon surrounding the nanospheres continued until the present day producing new Pb atoms inbetween the nanospheres but with a dramatically lower 207Pb/206Pb ratio. Thus there is what is called extreme isotopic heterogeneity (meaning dramatically different Pb isotope ratios) on a scale of only nanometers in the zircon between the Pb in the nanospheres and the Pb in the surrounding zircon. Its not surprising that people have been discovering that age dating of zircons can be less reliable than had been thought because its now clear that in attempting a measurement with a more conventional technique, more Pb nanospheres or less might be analysed, so giving variable results. Our new method of recognising and analysing Pb nanospheres individually can overcome the ambiguities and can give the age of metamorphism as well as the age of formation of the zircon, a big new plus for this new analytical method.
To find out more, see Lyon et al., (2019) Nanospheres in ancient zircon yield model ages for zircon formation and pb mobilization Scientific Reports 9:13702 doi.org/10.1038/s41598-019-49882-8